Predictions for the scalar partner of the LHC tetraquark X ( 6600 )

We consider how the recent CMS measurements of the masses and quantum numbers of X ( 6600 ) , X ( 6900 ) and X ( 7100 ) can help to reveal the internal structure of these apparent c c c ¯ c ¯ tetraquarks. The measured J P C = 2 + + quantum numbers of X ( 6600 ) are consistent with our previous prediction, and imply the inevitable existence of a lighter 0 + + partner X ( 6400 ) which also decays to J / ψ J / ψ . There are strong indications for this scalar partner in the recent CMS data fits, which include a Breit-Wigner peak with mass around 6400 MeV. We provide predictions for the masses and decay properties of the scalar and other partner states, which are key experimental tests to discriminate between quark and diquark models. Dedicated experimental scrutiny of this mass region is therefore required, to establish an S-wave multiplet of c c c ¯ c ¯ states (the first of its kind for tetraquarks), leading to a breakthrough in exotic hadron research and our understanding of heavy quark exotics.